Remotely operated vehicles are used in various situations where unsafe or hostile conditions are present. For example, bomb disposal, fire rescue, chemical spills, and military combat are just a few uses in such situations. Such systems typically include a remotely controlled or robotic manipulator arm for carrying out various tasks. It is desirable that the manipulator arm be light and strong. A manipulator arm typically includes one or more manipulator joints, and one or more manipulator links, where the manipulator joint architecture is a critical element of the overall mechanical system. As used herein, a manipulator link refers to an interconnecting structural member extending between two manipulator joints or a structural member extending from one manipulator joint. Desirable features of a manipulator joint used in field operations include a rugged drive mechanism, environmentally sealed housing, joint overload protection, and joint position feedback.
Unlike fixed manipulator arms, for example those used in a factory, where the loads are known and can be accounted for in the manipulator arm design, a manipulator arm on a remotely operated vehicle may encounter a wide variety of loads placed upon it. Further, the remotely operated vehicle may overturn or encounter an obstacle placing a very large load on the arm. The forces resulting from these loads are focused on the joints of the arm.
In order to prevent damage to a joint, typical manipulator joints may include a torque limiter. The torque limiter is designed to provide movement of the joint when a certain amount of torque is placed on the joint. This prevents the joint from being damaged and saves the cost of a repair or replacement of the joint.
Typical joints include a drive motor. Further, an encoder at the drive motor typically determines the position of the drive motor's shaft. The rotation of the drive motor shaft leads to a known movement of the arm that can be determined using the encoder's data and knowledge of the drive mechanism of the joint. For example, the power transmitted from the drive motor shaft to the joint may be geared such that ten drive motor shaft revolutions in a clockwise direction corresponds to five degrees of joint rotation in a counter-clockwise direction. In this example, a processor could calculate that encoder output data indicating thirty driveshaft revolutions in a clockwise direction corresponds to a joint rotation of fifteen degrees in a counter-clockwise direction. However, the processor coupled to the typical joint could not use this information to determine if the torque limiter in a joint had slipped due to overload.
A problem occurs when excessive torque is placed upon the joint of the manipulator arm and the manipulator joint slips from a known first position to a second position without a corresponding rotation of the drive motor shaft. In such a case, the second position is unknown by the motor encoder. In fact, if the slippage is not detected, and if the second position is calculated using data from an encoder of the drive motor, the calculated second position will not accurately reflect the true second position. For example, say that a weight resting on a surface was attached to an end of a manipulator arm via a flexible tether. A user desires to lift the weight above the surface and a rotation of a manipulator joint, at the opposite end of the arm, would affect such lifting. Assume however that the weight at the end of the arm would result in a torque at the joint that was greater than a maximum torque set for the joint's torque limiter. In such a case, the drive motor shaft would rotate and cause the joint to proportionally rotate until all of the slack was removed from the tether and tension was added to the tether. Up to this point, the joint's position is determinable given the encoder's data and the knowledge of the drive mechanism of the joint. However, just after this point, the drive motor will continue to rotate while the shaft remains stationary due to the slippage of the joint's torque limiter. As a consequence, because joint position is based solely on data from the encoder operationally connected to the drive motor shaft and knowledge of the drive mechanism of the joint, the actual position of the joint is unknown. Consequently, a reset or recalibration must be performed. Such a reset or recalibration may even be necessary when the manipulator arm has not been in use, or has been unpowered, because unknown loads may have caused slippage of the manipulator joint during handling or transport. Further, the operator may not be aware of the slippage caused by an overload condition and may not be able to properly initialize or control the manipulator arm.
Thus, there is a need in the electric manipulator joint field to create an improved and useful joint and encoder device to solve the problems mentioned above.